1. Field of the Invention
The present invention relates generally to the removal of environmentally unsafe materials from wastewater and chemical solutions. More particularly, the present invention relates to a method which uses a process reactor having three separate reaction stages to remove high concentrations of heavy metals from acidic wastewater and chemical solutions.
2. Description of the Prior Art
A variety of industrial activities generate wastewater that is highly polluted by heavy metals such as copper (Cu), nickel (Ni), zinc (Zn) and lead (Pb), Chemical Oxygen Demand (COD), Total Suspended Solids (TSS), and other organic matters. For general industrial activities the concentrations of heavy metals in wastewater streams range from low to moderate in the hundreds parts per million (ppm). For some industrial operations such, as pipe descaling, flushing and cleaning of heat exchange systems by using very acidic chemical solutions at a pH value below 1, the concentration of heavy metals in wastewater streams is very high in the range of thousands mg/l (or parts per million (ppm)). Wastewater streams containing heavy metals is considered environmentally hazardous and needs to be processed prior to being discharged into municipal wastewater systems to meet local regulatory discharge requirements. Increasingly stringent regulations on effluent discharge mandate efficient heavy metal removal techniques.
For the wastewater streams with low to moderate concentrations of heavy metals in the hundreds ppm, a variety of conventional and commercially available techniques have been employed to remove heavy metals either as primary, secondary or tertiary unit operations and processes to achieve the desired level of treatment. The techniques for removal of heavy metals from wastewater streams include air flotation, precipitation by neutralization, flocculation and coagulation, adsorption and ion exchange.
However, conventional clean up technologies used in heavy metal removal are either inadequate in meeting the disposal limits or too expensive in terms of chemical, operation and maintenance (O&M) costs for an acidic waste stream with high concentrations of heavy metals in the level of thousands ppm. For an example, the cost associated with removing heavy metals from acidic wastewater generated by industrial power plant heat exchangers and shipboard seawater cooled heat exchanger pipe flushing cleaning operations is very expensive since the wastewater normally contains over 2,000 ppm, 800 ppm, 100 ppm and 10 ppm of Cu, Ni, Zn, and Pb, respectively.
The following conventional and commercially available methods used in removing heavy metals from various wastewaters have their limitations and are inadequate or too expensive to treat very acidic wastewater of pH less than one and very concentrated heavy metal wastewater at the level of thousands ppm.
The hydroxide precipitation/clarification process adjusts the pH of the wastewater with an alkaline reagent to reduce the solubility of the dissolved metals. Settling occurs and the resultant metal hydroxide precipitates are removed. Sodium hydroxide (NaOH) is most commonly used for hydroxide precipitation. Sulfide precipitation, which precipitates metals as sulfides instead of hydroxides, achieves low levels of metal solubility in highly chelated wastewater streams. Since each metal hydroxide has a characteristic solubility that is dependent on pH, the optimal pH for precipitating all regulated metals to the discharge level from a wastewater stream is very difficult to obtain.
Dissolved air flotation (DAF) technology employs the combination of air flotation and flocculation to separate the metals from the wastewater stream. During metal precipitation, ferrous sulfate, sodium hydrosulfite, aluminum sulfate, soda ash or sodium dithiocarbamate (DTC) is added to provide co-precipitation for the removal of metals from chelated wastewaters. While DAF technology is an effective metals removal method, this process has a metal removal rate of 70% (100 ppm to 30 ppm), generates a significantly larger volume of sludge compared with hydroxide treatment, needs a large foot print, and is operation and maintenance (O&M) intensive.
The electro-coagulation (EC) process passes a controlled electrical current through industrial wastewater at a monitored pH level to aid in the removal of contaminants including heavy metals in very low concentration levels. It is impractical to use this technology to treat wastewater having very high metal concentration levels of thousands ppm at a low pH of less than one.
The ion-exchange process is widely used in the field of wastewater treatment to remove dissolved metal ions in very low concentration levels. Wastewater is passed through an ion-exchange which consists of a porous bed of organic resin with an ion exchange function. Cationic and anionic ion exchangers react with cations and anions, respectively, for removal or recovery of heavy metals. These can be placed either in series or in mixed beds. The pH is critically controlled to ensure that the pH is within the operating range of the resin. Most O&M problems for ion exchange applications relate to resin fouling and the frequency of regenerating the resin beds. This technology is not practical for direct treating a wastewater stream with low pH (less than 1), very high metal concentration of over thousand ppm, and a high COD of 10,000.